The present invention is related to the field of subsurface to surface data transmission involving reservoir management, electric wireline well logging tools, and other subsurface logging devices. More specifically, the present invention is related to systems for communicating data signals (e.g., logging data) from locations disposed deep within wellbores to a surface-based transceiver, such as a recording system or a relay repeater (wireless or other transmission means) located at the earth's surface.
Those of ordinary skill in the art will be quite familiar with the use of electric wireline well logging tools, which are used to make measurements of certain properties of earth formations penetrated by wellbores. The measurements can assist the wellbore operator in determining the presence, and quantity if present, of oil and gas within subterranean reservoirs located within the earth formations. Well logging tools known in the art are typically extended into the wellbore at one end of an armored electrical cable. The cable can include at least one, and commonly includes as many as seven, insulated electrical conductors surrounded by steel armor wires. The armor wires are included to provide abrasion resistance and tensile strength to the cable. The cable supplies electrical power to the logging tools and provides communication channels for signals sent between a transceiver located in the proximity of the logging tools and another transceiver usually located near the wellbore at the earth's surface.
Logging tools known in the art can provide many different types of measurements of earth formation properties, including measurements of electrical resistivity, natural gamma-ray radiation intensity, bulk density, hydrogen nucleus concentration and acoustic travel time, among countless others. Another class of logging tools, generally called “imaging” tools, provide finely detailed measurements, meaning successive measurements can be made at axial and radial spacings of as little as several hundredths of an inch, of resistivity and acoustic pulse-echo travel time in order to generate a graphic representation of the wall of the wellbore, or of one or more physical parameters of the rock.
It is known in the art to digitize analog well logging measurement data. This involves converting the analog output signals from each logging tool into a stream or sequence of binary digital values composed of “words” comprised of a plurality of digital bits, bits being signal levels representing numerical (binary) “ones” and “zeroes”. For example, digital words can represent the numerical values of the measurements sampled at spaced apart time intervals. The measurements are then typically transmitted to the surface transceiver as a series of digital bits arranged in a predetermined pattern. The significance of the predetermined pattern will be further explained.
It is generally beneficial to the wellbore operator to be able to combine as many different types of logging tools as is practical into one continuous instrument package (generally called a “tool string”). The benefit to the operator is the reduction of the number of times logging tools must be extended into the wellbore, likely saving a considerable amount of operating time. Combining a large number of measurements generally requires that large amounts of data be sent to the surface.
A particular problem in combining large numbers of measurements in the tool string is that the large amount of signal data to be transmitted can cause the required quantity of signal data per unit of time (e.g., bits per second) to exceed the signal carrying capacity (bandwidth) of the transmission medium, such as a cable. This problem is particularly acute when imaging tools are included in a tool string because of the very fine measurement both in time and space, and consequently the large increase in the amount of signal data, of imaging tools relative to other types of tools.
The cable—indeed, any transmission medium—may have limited signal transmission capacity for any of a number of reasons, some controllable, some not. For example, there may be a restriction on the external diameter of a cable for reasons related to safety of the wellbore and personnel near the wellbore. A practical limit for the diameter of seven conductor cables known in the art can be about 19/32 of an inch. Limited cable diameter provides cables generally having electrical properties which limit their frequency response to less than about 200,000 Hz (200 kHz). The frequency response of a cable is limited, and cables typically exhibit a “transfer function” roll-off at higher frequencies, as would be understood by those of ordinary skill.
It is known in the art to increase the effective signal data carrying capacity of a logging cable by encoding the binary digital signals using various types of encoding methods. The encoding methods constitute the previously described “predetermined pattern” of digital bits. Quadrature amplitude modulation (“QAM”) telemetry can be used to transmit well logging data to the recording system, and one type of QAM telemetry system used for well logging tools is described, for example, in U.S. Pat. No. 5,387,907 issued to Gardner et al. QAM telemetry includes conversion of groups of digital bits (“bit groups”) in the previously described predetermined pattern into two-dimensional symbols, each comprising coordinates corresponding to the bit values in each bit group. The coordinates are converted to in-phase and quadrature analog signal amplitudes which are used to drive a specialized analog signal modulator. The modulator controls the output amplitude of a signal carrier generator. The modulated carrier is applied to the logging cable. Signal data are recovered in the recording system by extracting the amplitude values from the modulated carrier and reconverting them to digital bits.
A drawback to QAM when used in wireline well logging tool signal telemetry is that precise recovery of the data signal impressed onto the carrier requires a complex and expensive signal demodulator to precisely recover the amplitude and phase of the carrier.
Clark et al. (2001 publication—Schlumberger WO 01/49001 A1) applies to the downhole environment discrete multi-tone modulation (DMT) techniques used in asymmetric digital subscriber line (ADSL) applications commonly used in surface telephony and the like. With ADSL techniques it is possible to accomplish a large data throughput of several million bits per second on a standard twisted-pair telephone line. Each tone is used as a separate carrier. The amount of bits transmitted per each carrier per unit time increases with higher signal to noise ratio detected in the transmission of each carrier. This application has a perceived drawback of wasting the transmission media's power dynamic range available with carrier energy which effectively does not transmit any data.
Those of ordinary skill in the art will appreciate that Directional Drilling Systems (DDS), Measurement While Drilling (MWD), Logging While Drilling (LWD), Electric Wireline Well Logging (EWL), and permanent and semi-permanent subsurface devices (PS) are respectively employed during the drilling, post drilling and production phases of oil and gas well developments. These devices are employed to assist the wellbore operator in determination of the direction of the wellbore while drilling, properties of the earth formations, both while drilling and shortly thereafter, and still later in determining or controlling important production characteristics when the well is placed into production. In all these applications there is a need to communicate between the subsurface and surface devices as well as between the subsurface devices in the wellbore.
The medium for communication between the devices will vary depending on the application. Electrical conductors or optical signals may be used for EWL or PS, while acoustic, hydraulic pressure or flow pulses or electromagnetic energy may alternatively be used for MWD or LWD, and furthermore, electromagnetic waves for DDS applications, for example. Each of these will have a widely varying transmission bandwidth and each application will have a theoretical maximum data transmission speed based on bandwidth and signal-to-noise characteristics allowed by the transmission medium, according to Shannon's formula.
Many of the associated methods disclosed for improving subsurface to surface communications speed in oil and gas exploration business have been directed toward conventional EWL cable, and there have been adaptations of methods that were earlier disclosed for use in telecommunication applications. Some of the more recent methods disclosed include: adaptive filtering, Quadrature Modulation (QAM), discrete Multi-tone (DMT) and carrier-less amplitude and phase (CAP) methodologies.
U.S. Pat. No. 5,473,321 to Goodman et al. teaches the use of an adaptive communication system for transmitting data on conventional EWL cable. The method employs transmitting a periodic pseudo-random training sequence to effectively initialize and optimize an adaptive filter equalizer to improve communication between a surface device and subsurface logging instrument. The subsurface instrument transmits a predetermined training signal to the surface device until the surface receiver has acclimated itself to the transmission characteristics of the EWL cable by adaptively configuring the filter-equalizer, thereby enabling the surface receiver to more accurately interpret the data received, reducing the effects of the signal attenuation, noise, or other distortions.
U.S. Pat. No. 5,387,907 to Gardner et al. discloses a QAM method for increasing the data transmission capability of a conventional EWL cable. QAM employs transceivers that modulate and demodulate the amplitude of sine and cosine carrier signals to encode transmitted data.
WO 01/49001 to Clark et al. discloses a DMT method for increasing the data transmission capability of a conventional EWL cable. DMT is an extension of QAM, as it is a multi-carrier QAM transceiver implementation. The transmission bandwidth of the EWL is divided into multiple carrier frequencies, each transmitting QAM encoded data. The amount of data placed onto a particular carrier frequency can be adjusted or even made zero if an unacceptable level of noise is encountered at that frequency. Reception of the acquired data is similar to the transmission and involves a reciprocal demodulation method. The DMT multi-carrier method enables increased data transmission and an improvement in flexibility when coping with noise, compared to single-carrier frequency QAM.
U.S. Pat. No. 5,504,479 to Doyle et al., incorporated herein by reference, discloses a CAP method for increasing the data transmission capability of a conventional EWL cable. CAP is similar to QAM in that it employs phase and amplitude modulated orthogonal signals to encode and decode data. CAP, as used for the telecommunications market, is described, for example, in “Contribution T1E1.4/90-154, Carrier less AM/PM”, by Sobrara et al., presented to American National Standards Institute (ANSI) T1E1.4 Technical Subcommittee Working Group, 1990, where it also illustrates the QAM may be derived as a subset of CAP. Because CAP does not employ a carrier signal, is not subject to performance degradation due to errors in recovering the carrier (and does not use power to transmit a carrier); nor does CAP require a complex and expensive demodulator. CAP also lends itself to a digital implementation, as known in the art. Establishing a high speed communication modulation standard for the internet, for example, has led to a significant competitive debate particularly between the proponents of the front-runner methods of DMT and CAP. Each technology has demonstrated operational success bringing certain advantages as mentioned, but as presently defined within the telecommunications industry, the two methods are significantly different and not compatible.
Unlike the telecommunications market, the issue of compatibility is relatively insignificant for the communication applications described herein, since the transmission is typically between devices of a proprietary nature, where both ends of the communications means are controlled by the same user. In the event communication with a third party device is desired, a communication bridge between the devices could be formed using a common protocol such as Ethernet, for example. Also, unlike the telecommunications market, the cost of the communication device is of only minor concern. For example, if the communication method required several additional components, such as DSP's (digital signal processors), the incremental cost would be a minor fraction of the cost of the combined investment in the communication and associated devices employed. By far, the robustness of the communication, the potential value of an increased data rate and the capability to operate in the adverse subsurface environment are the far more important objectives.
A perceived drawback to CAP systems such as proposed in the Sobrara et al. reference is—that the system is best suited for telecommunications applications (e.g., a public telephone network), rather than for well logging applications. The system described in the Sobrara et al. reference can have inadequate ability to compensate for changes in signal timing that can particularly occur in wireline signal telemetry because of changes in electrical signal transmission properties of the cable and in signal timing generator changes in the well logging tools. Timing generator changes in the tools and electrical property changes in the cable can result from variation in the ambient temperature of the cable and the electronics in the well logging tools, as they are extended into and withdrawn from the wellbore. CAP telemetry system for use in wireline well logging which can compensate for timing generator changes and electrical transmission property changes in the logging cable.
These perceived shortcomings were addressed in U.S. Pat. No. 5,504,479 to Doyle et al., entitled “Carrierless Amplitude and Phase Modulation Telemetry for Use in Electric Wireline Well Logging”. The Doyle et al. '479 patent is hereby incorporated by reference herein in its entirety. This telemetry system proposed in Doyle et al. was devised for use in wireline well logging which can compensate for timing generator changes and electrical transmission property changes in the logging cable. A possible disadvantage of this method is the vulnerability of a single channel operation. If this channel becomes corrupted by noise the entire telemetry system is shut down.
Further background is included in the following references and incorporated herein by reference: “Contribution T1E1.4/90-154, Carrierless AM/PM”, by Sobrara et al, as presented to American National Standards Institute (ANSI) T1E1.4 Technical Subcommittee Working Group, 1990; Bracewell, R., “The Fourier Transform and Its Applications”, 3rd ed., New York: McGraw-Hill, pp. 267-272, 1999; and “Digital Communication”, third edition, Springer, Section 5.4, pp. 164-184, 2004.